Frequency offset estimator for upstream cable signals

Abstract

In a DOCSIS transmission network, it is necessary for an upstream receiver to estimate and correct for errors in the carrier frequency of the transmission based upon observations of a preamble sequence. This task is complicated by the fact that microreflections in the upstream channels cause intersymbol interference, which tends to bias the frequency estimation circuit. An economical, ISI-immune algorithm is provided for estimating the carrier frequency of upstream transmissions. The arrangement improves the reliability of the frequency estimates in the presence of ISI, thereby reducing the overhead and increasing the throughput of the upstream channels.

Description

[0001]This application claims under 35 USC 119 the benefit of priority from Provisional Application 61 / 383,810 filed Sep. 17, 2010.[0002]This invention relates to a method of estimating the frequency offset of an RF or IF signal of an upstream cable signal.BACKGROUND OF THE INVENTION[0003]The DOCSIS (Data Over Cable Service Interface Specification) standards define a protocol governing the transmission of data over hybrid fiber-coax networks. In contrast to the broadcast model used in traditional analog television services, many modern communications applications supported by DOCSIS networks require two-way data transfer.[0004]As described in more detail hereinafter, a modern cable network consists of two main classes of devices: Cable Modems (CM) and Cable Modem Termination Systems (CMTS). The CM is a low-cost, mass-produced device that is used by each customer to connect to the network, while the CMTS is a large and complex piece of equipment residing on the cable operator's premi...

AI Technical Summary

Benefits of technology

[0026]It is one object of the present invention to increase the efficiency of the upstream channels through the use of a fast-converging and robust carrier frequency recovery algorithm for use in a DOCSIS upstream receiver.

[0043]The present invention therefore involves a novel algorithm for data-aided carrier frequency estimation in upstream cable channels. Due to its differential detector based structure, the algorithm is economical to implement, while still providing a high level of performance. An additional aspect of the invention includes a technique for mitigating the effects of upstream channel ISI through the selection of preambles matched to the circuit used for frequency estimation. This scheme improves the reliability of the frequency estimates in the presence of ISI, thereby reducing the overhead and increasing the throughput of the upstream channels.

Problems solved by technology

The CM is a low-cost, mass-produced device that is used by each customer to connect to the network, while the CMTS is a large and complex piece of equipment residing on the cable operator's premises.

Due to the recent explosion of two-way communications applications such as voice-over-IP and video conferencing, there is significant competition for bandwidth on the upstream channels.

In general, the task of the receiver in a communication system is complicated by a number of physical layer distortions which are commonly present in the communication signal.

Such distortions generally arise from a lack of synchronization between transmitter and receiver, nonidealities in the transmission medium, or mismatches between the hardware components used to construct the transmitter and receiver.

Unless compensated for in the receiver, these distortions tend to impair the performance of the communication system.

The amount of published work in the area of frequency estimation specifically for DOCSIS upstream channels is relatively limited.

Improperly terminated CMs in a DOCSIS network often reflect multiple copies of the transmitted upstream signals back to the CMTS.

The result is significant intersymbol interference (151) in the received signal, which tends to have a biasing effect upon the previously discussed carrier frequency estimators.

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